Non-aqueous secondary batteries including, for example, lithium secondary batteries have a high voltage and a high capacity, and thus are widely used as power sources for mobile phones, and also for various other portable devices such as smartphones and tablet terminals in recent years. They also have found medium- and large-sized applications including power tools such as electric tools, electric cars, and power-assisted bicycles.
In a standard charging method commonly used for lithium secondary batteries, when 1 C is the current value with which a fully charged battery can be discharged in one hour, constant-current (CC) charging is performed with a current of about 0.7 to 1 C until a predetermined end-of-charge voltage is reached. After the end-of-charge voltage has been reached, charging is switched to constant voltage (CV) charging in which the charge current is decreased so as to maintain the end-of-charge voltage.
Meanwhile, there is also a need to complete the charging of batteries as fast as possible. For example, in the case of lithium secondary batteries for mobile phones, conventional lithium secondary batteries for mobile phones can be brought into a fully charged or nearly fully charged state by being charged with a current value of 1 C or less for about 2 to 4 hours. However, with the sophisticated functions of devices, such as mobile phones, to which lithium secondary batteries are applied, and with the wide spread use of smartphones and tablet terminals, which have a larger size than mobile phones, lithium secondary batteries are required to have a higher capacity. Accordingly, charging with a current value that is about the same as conventionally used current values may increase the time required to reach a fully charged state to be longer than the practical range. For example, for a battery having a high capacity exceeding 1500 mAh, the current value corresponding to 1 C is relatively large. In order to prevent heat generation resulting from charging at a large current, it is required to charge with a value as low as about 0.7 C, and as a result the time required for charging will be increased. Thus, in order to avoid this, there is a need to develop a technique for enabling charging with a larger current value to achieve a higher capacity, while reducing the time.
To meet this need, there have been proposed, for example, a method of enhancing fast-charging characteristics by using a plurality of positive electrode active materials in combination (Patent document 1), a method of increasing the output (improving the load characteristics) and enhancing the fast-charging characteristics by using a lithium-titanium composite oxide for the negative electrode (Patent document 2), and a method of ensuring favorable battery characteristics even after fast charging by adding an insulating inorganic oxide filler to the negative electrode, separately from an active material (Patent document 3).
From the viewpoint of simply enhancing the load characteristics of lithium secondary batteries, it has been reported that the use of SiOx having a structure in which Si ultrafine particles are dispersed in SiO2 as a negative electrode active material is effective (Patent documents 4 and 5).
On the other hand, for devices, such as an electric tool, from which a battery pack is removed from the device body and charged, there has been proposed a method in which an abrupt temperature rise of the battery pack due to fast charge is suppressed forcibly, thereby allowing charging to be performed with a current exceeding 1 C and thus reducing the charging time (Patent document 6).